It seems to me that the one thing that would clearly be wrong would
be to allow xml:base but not use it to interpret relative URIs.
If there are no relative URIs, then there is no occasion to use the
value of xml:base, and consequently no need to disallow it. But if I
understand the proposal correctly, there will be no relative URIs in
the envelope, but there may be in the content (which is presumably
nested inside the envelope elements?). In that case, either xml:base
should be disallowed in the envelope or it should be used correctly
to resolve relative URIs in the content.
-- Richard